Still, judging by the shape of the curve you might consider interpolation if you can't find anything else. Keep in mind we're talking about seriously cryogenic temperatures at those low pressures, probably less than 10K!

Right. I ran into the same problem as the NIST web book is typically my go-to source as well.

Extrapolating may provide a reasonable estimate. The point of my query is that for supersonic wind tunnels you can achieve static temperatures that low and if you don't pay attention to the saturation temperature of your gas, you can see liquefaction occur. I am planning on writing a homework problem based on this but was trying to make sure the data was readily available first. Maybe I'll just stick to helium. That is more commonly used anyway.

Again, you can easily get down into that range in a wind tunnel if you have a high enough Mach number. For example, in a Mach 10 wind tunnel, if you started with your reservoir at room temperature (300K) you would have a static temperature in your test section of 14K. This is why wind tunnels operating at high Mach numbers must have the flow preheated.

It looks like Helium's saturation temperature will be very low, less than 2K if we believe the quick and dirty power series fit interpolation from Excel.

Looking at the data sets side by side Nitrogen is more difficult to predict than I expected because it's unclear how its properties change at very low pressures. It seems that it would have to asymptotically approach (0.000 K , 0.000 atm) but a simple power series fit doesn't do a very good job representing that... so basically don't listen to me ;-)